Integrated gasification combined cycles known as IGCC cycles employ a gasifier to gasify coal into a fuel gas containing hydrogen, carbon monoxide, carbon dioxide and possibly methane. The fuel gas is fed to a gas turbine to generate electrical power. Oxygen for the gasification of the coal is supplied by an on-site air separation plant that also produces nitrogen. In many situations most or all of the nitrogen is introduced into the head end of the gas turbine combustors for NOx control and for power augmentation.
Gas turbine fuel delivery systems are designed to provide the gas turbine with a controlled fuel energy input measured in BTU/hour and based upon the lower heating value of the fuel. This is accomplished by supplying fuel to the gas turbine combustor at a specific pressure via control valves, orifices or nozzles. For a given fuel system design, the turbine energy input is controllable with a specific set of control valves, orifices and nozzles as long as a quantity [LHVg/(MWgTg/28.96)0.5] known as the modified Wobbe index remains constant. For this reason, the modified Wobbe index is considered a measure of fuel interchangeability. That is, fuels with various LHVg (lower heating values), MWg (molecular weights) and Tg (gas temperature) can be accommodated by a given fuel delivery system, as long as the modified Wobbe index closely matches the design value. Typically, gas turbine manufacturers allow for ±5-10 percent deviations from the design modified Wobbe index. Of course, other constraints may also be imposed by the turbine manufacturer, such as LHVg≧200 BTU/scf to ensure proper combustor operation or Tg≦550° F. to protect materials of construction.
Given the fact that the composition of gas turbine fuels can vary, for example, natural gas, it has long been known to adjust the modified Wobbe index of the fuel to meet design specifications with the addition of nitrogen or other inert gas. For example, in GB 1 366 484, the Wobbe index of natural gas from the North Sea is lowered by the introduction of nitrogen to meet specifications involving natural gas from continental sources.
U.S. Pat. No. 6,896,707 relates to a situation in which a turbine is used in connection with a gas to liquids facility that employs a Fischer-Tropsch process. It is advantageous to supply fuel to the gas turbine that is derived from a tail gas produced by the Fischer-Tropsch process. However, this tail gas is not always available and hence natural gas has to be substituted in some instances. Since the tail gas includes hydrogen, carbon monoxide and carbon dioxide, it does not have as high a heating value as that possessed by natural gas. Hence, the use of these two fuels presents compatibility problems related to their differing Wobbe index. In order to solve this problem, the Wobbe indices of the natural gas and or the tail gas are adjusted so that both are compatible. In the specific embodiment, the low Wobbe index fuel can be increased by removing the carbon dioxide and the high Wobbe index fuel can be decreased by adding carbon dioxide. In another example, a natural gas stream is blended with nitrogen to provide a blend with a lower Wobbe index than the starting natural gas.
In yet another method of adjusting the Wobbe index of fuel, U.S. Pat. No. 6,082,092 discloses a control system for a gas turbine in which the Wobbe index is computed for the fuel and a feedback signal is provided to change the temperature of the fuel supplied to the gas turbine combustor to change the Wobbe index of the fuel. In yet another method of adjusting Wobbe indices of fuel, a dry fuel gas is saturated with water and the Wobbe index is adjusted by adjusting the flow of the recycled water to the saturator.
One advantage of IGCC plants for coal-based power generation is that equipment can be installed for carbon dioxide removal in a very cost effective manner. The need to remove carbon dioxide emissions can be regulatory in nature. However, it also may be desirable to remove carbon dioxide for its value as a product or an agent in other processes that may take place near the IGCC facility. Such processes include enhanced oil recovery operations in which the carbon dioxide is injected down hole. It therefore can become valuable to retrofit the IGCC facility with a water gas shift reactor in which the carbon monoxide content of the stream is reacted in a known manner with water to produce additional hydrogen and carbon dioxide. The carbon dioxide can then be conventionally removed, by such well-known techniques as those involving absorption towers.
When adding the carbon dioxide removal system, the problem of fuel compatibility will arise. In case of a retrofit installation, the gas turbine will have to be modified to accept a higher Wobbe index fuel due to the change in composition of the fuel.
Further complicating the problem is that NOx emissions are currently regulated and in current IGCC plants a diffusion combustor is used in which nitrogen is added to the head end of the combustor as a diluent to lower flame temperature. Nitrogen is produced by an on-site air separation plant that also produces oxygen for the gasifier. In case of retrofit installations, the most expeditious and cost effective solution is one that will not involve a modification of the air separation plant and its controls.
As will be discussed, the present invention provides a method of adjusting the modified Wobbe index of a fuel produced in an IGCC installation that has varied from an original design level in a cost effective manner.